I haven't compared the two, but as far as i know those are the rules for CS6 and i assue that the same goes for CS5:

DVD, 4:3 menu = 18 buttons

DVD, 16:9 menu = 36 buttons

Blu-ray menu = 32 buttons

However, be aware of that when you preview the DVD/Blu-ray you can only use the first 18 buttons on any menu. The other buttons cannot be used when previewing in Encore, but they will be usable and fully functional on the burned disc.